Gas plasmas are widely used in a variety of integrated circuit (IC) fabrication processes, including plasma etching and plasma deposition applications. Generally, such plasmas are produced within a processing chamber by introducing a low-pressure process gas into the chamber and then directing electrical energy into the chamber for creating an electrical field therein. The electrical field creates energetic electrons within the chamber which ionize individual process gas molecules by transferring kinetic energy through individual electron-molecule collisions. The electrons are accelerated within the electric field in the processing chamber for efficient ionization of the gas molecules, and the ionized molecules of the process gas and free electrons collectively form what is referred to as a plasma or plasma discharge.
There are a variety of different methods for producing a plasma within a processing chamber. For example, a pair of opposing electrodes might be oriented within the chamber to capacitively couple energy to the plasma. Inductive coupling processes are also popular, and are particularly desirable for their capability of producing a high-density plasma. Inductively coupled plasmas are formed generally utilizing a shaped coil or antenna positioned with respect to the process chamber to inductively couple energy into the processing chamber and thus create and sustain a plasma.
For example, in one particular design for an inductively coupled plasma (ICP) system, an inductive coil or antenna is solenoidal in shape and is wound around the outside of the processing chamber to inductively couple RF energy to the plasma through the chamber sidewalls. In such a system, a portion of the processing chamber is fabricated from a dielectric substance, typically quartz or ceramic, through which the inductive energy from the coil may pass. The high-density plasma developed by such a system is particularly suitable for etching a substrate. Various other ICP systems are also known for use in plasma processing, such as systems which utilize flat coils and inductively couple energy to the plasma through a dielectric window in the top of a processing chamber.
As noted, one particular use for an inductively coupled plasma is sputter etching of a substrate. During processing of a substrate in the formation of integrated circuits, sputter etching is a technique that is utilized to remove a layer of undesired material from an exposed surface of a substrate. Such an etching process is referred to as a pre-clean etch. For example, the surface of a silicon substrate might be pre-cleaned with a sputter etch process prior to the deposition of a metal thin film on the surface. The purpose of the etch is to remove the native oxide and other contaminants from the silicon or other conductive surface in order to ensure a low resistance connection to a metal thin film to be deposited thereon. Typically, for a silicon substrate, the upper substrate surface might be predominantly silicon dioxide (SiO.sub.2), an insulator, with small holes or vias formed through the upper surface to the silicon or other conducting surface beneath. Generally, an IC substrate will include multiple material layers which form the conductive and insulative surfaces of the integrated circuits. For example, a silicon layer or surface would occur at the base contact level of the substrate, and another conducting surface, such as an aluminum or titanium nitride surface, would occur at the via level in a multilevel substrate application.
The physical mechanism in a sputter etch cleaning process is the bombardment and resulting removal or "sputtering" of particles from the substrate surface. By using low energy (100-300 eV) argon ions from the gas plasma, a "soft" sputter etch is produced. The substrate is electrically biased to attract the positive plasma ions, to bombard the surface layer, and the sputtering of the substrate surface etches away a small amount of the surface layer. A typical and desirable etch rate for SiO.sub.2 in such a process is approximately 300-500 .ANG./min., while the etch rate for a metal oxide is lower. For example, the etch rate for aluminum oxide Al.sub.2 O.sub.3 would generally be about one-fourth of the etch rate for SiO.sub.2. For a plasma sputter etch, a high density plasma is provided by the ICP source which utilizes an RF power supply or source. A substrate bias is provided by a second RF source electrically coupled to the substrate. Independent RF sources for the plasma and substrate bias provide independent control of the plasma ion current (plasma density) and the ion energy (substrate bias) delivered at the substrate surface being sputter etched. The product of the plasma ion current and ion energy determines the etch rate for a particular material to be sputter etched.
For silicon substrates, which are commonly used in IC fabrication, the substrate will comprise a significant amount of SiO.sub.2. Therefore, when large amounts of the oxide (SiO.sub.2) layer are sputtered during the pre-clean etch, the etched SiO.sub.2 particles pass through the high density plasma. The particles are then broken down into activated oxygen in the form of oxide radicals or free oxygen which are then adsorbed onto the processing chamber walls. The chamber walls are formed of a dielectric material. Such accumulation of oxygen in the chamber walls will detrimentally affect the efficiency of the processing chamber and the sputter etch process for subsequent substrates being processed. Specifically, during subsequent etches of additional substrates, the adsorbed oxygen is evolved from the walls, and the evolved or reflux oxygen is activated in the plasma and suppresses the etch rate, which reduces the overall efficiency of the processing system. The oxygen levels from such etching have been observed to remain in the walls of the chamber over long periods of time, and they will generally be at the same level even after the chamber is not used for such etching processes for a couple of days. Therefore, activated oxygen in the plasma etch process is undesirable. However, since oxide and oxygen-containing layers of the substrates are being etched, such oxygen reflux from the chamber walls in subsequent etches is a problem which must be addressed.
Accordingly, it is an object of the present invention to improve the efficiency of a sputter etching processing system. It is specifically an objective to improve a sputter etching system which is utilized to pre-clean an oxide layer, such as silicon dioxide, from a series of substrates.
It is a further objective of the present invention to reduce the amount of oxygen evolved from the wall of a processing chamber during sputter etching of a substrate and subsequent sputter etching of additional substrates.
It is another objective of the present invention to reduce the effect of any oxygen evolved from the walls of a processing chamber during sputter etching.
It is still another objective of the present invention to increase the overall etch rate and processing efficiency of a sputter etch processing system.